#include <iostream>
#include <fstream>
#include <iomanip>

int main() {
	std::ifstream in("in.txt");
	std::ofstream out("out.txt");
	double x, y;
	double x_first, y_first;
	double x_prev, y_prev;
	double sum = 0;
	in >> x_first >> y_first;
	in >> x_prev >> y_prev;
	while (in >> x >> y) {
		sum -= (x_first - x_prev) * (y_first - y) - (x_first - x) * (y_first - y_prev);
			x_prev = x;
			y_prev = y;
	}
	out << std::setprecision(7) << sum / 2;
	return 0;
}
